The present invention relates to work supported in part by Grant No. ECS 82-00312 of the National Science Foundation.
The present invention relates, in general, to optical microscopy, and more particularly to near-field scanning for high-resolution imaging.
With the advance of submicron technology, the need for a microscope using light for use in the microanalysis of materials has steadily increased. Although various devices, such as electron microscopes, are available for detecting objects with a very high degree of resolution, such prior devices have required that the samples to be observed must be inserted into a vacuum or must be subjected to ionizing radiation. These techniques result in serious damage or destruction of the sample, and, particularly when biological material is being studied, such techniques have been unsatisfactory.
Nondestructive viewing of samples can be obtained with presently available technology, using visible light in two different ranges. At the lower end of the scale, fluorescence spectroscopy, coupled with chemical methods, can be used to determine on a statistical basis the dimensions between objects that are up to about 80 Angstroms apart. At the upper end of the scale, light microscopy, when used in the fluorescence mode, can be used to determine dimensions as small as about one-half the wavelength of the light that is used; that is, down to about 2,500 Angstroms. However, separations between objects, or feature dimensions, of between 80 Angstroms and 2,500 Angstroms are inaccessible when visible wavelengths are used. The ability to determine such dimensions using light microscopy would be very important since, unlike electron microscopy, samples could be studied in their natural environment without resorting to high-vacuum conditions and without the risk of damage. Such actuability would be particularly useful in biological applications where clinical testing or chemical mapping are to be done.
As discussed in copending application Ser. No. 520,041, now U.S. Pat. No. 4,662,747, filed Aug. 3, 1983, and assigned to the assignee of the present application, visible radiation can be transmitted in useful amounts through submicron apertures which are on the order of one-sixteenth of the wavelength of the incident radiation and the radiation emanating through the aperture will be the geometric projection of that aperture. This feature is essentially independent of the wavelength of the incident light. Further, when an aperture is very close to or in contact with an object which is to be imaged, radiation from the object passing through the aperture is the geometric projection of that part of the object which falls within the projection of the aperture. The radiation pattern produced by light passing through a submicron aperture becomes more diffuse as a result of the changing angular distribution of the radiation, which occurs in the Fresnel region. Eventually, a distance is reached where the angular distribution of the radiation pattern becomes constant as a function of distance, so that further motion does not change the shape or size of the pattern. This is known as the far field of the light pattern. Between the aperture and the beginning of the Fresnel region, the radiation is collimated and essentially projecting the shape of the aperture. This region is known as the near-field, and extends for a distance from the surface of the material on which the aperture is formed equal to about one-half the diameter of the aperture.
Over a decade ago, the principle of super-resolution microscopy was demonstrated at microwave frequencies by E. A. Ash and G. Nicholls ("Super-Resolution Aperture Scanning Microscope", Nature 237, p.510, 1972). In their experiment, a grating of 0.5 mm periodicity was imaged with an effective resolution of one-sixtieth the wavelength of the incident radiation. However, until the work described in the above-described copending application Ser. No. 520,041, the applicants therein were unaware of any published attempts to extend this technique to the visible region of the spectrum. Not only did the minute physical dimensions of the optical near-field demand aperture fabrication and micropositioning technologies beyond those available at the time of the Ash, et al, publication, it was also not known whether the results of the microwave experiment could be extended to the visible region. U. C. H. Fischer ("Optical Characteristics of 0.1 um Circular Apertures in a Metal Film as Light Sources for Scanning Ultramicroscopy", J. Vacuum Science Technology, B.3, p.386, 1985) discloses results obtained by scanning a subwavelength aperture over a second, larger aperture. However, the results obtained by that device are difficult to interpret, since the opacity of the metal films used therein was not large, so that the apertures were poorly defined. In addition, coherent monochromatic illumination was used at a grazing incidence, so that a series of standing waves may have been generated to produce the reported results.
A near-field imaging system for use in the far infrared is described by G. A. Massey, et al. ("Subwavelengths Resolution Far-Infrared Microscopy", Applied Optics 24, p.1498, 1985). Although this system may find many applications in the detection of heat transport on a microscopic scale, for example, it does not provide resolution capabilities on a submicron scale. Pohl, et al. ("Optical Stethoscopy: Image Recording With Resolution .lambda./20", Applied Physics Letters 44, p.652, 1984), have developed a system for superresolution microscopy, but the sizes and the structure of the apertures used were not characterized. Furthermore, the manufacturing techniques presented in that article present considerable challenges in the attainment of reproducibility.
The foregoing attempts to implement a near-field scanning technique attest to the difficulty of obtaining success. To further demonstrate the technical challenges inherent in this form of microscopy, the transmission of light through a slit of infinite length in a screen of finite thickness was calculated. The results demonstrated that the radiation passing through such an aperture remains collimated to a distance of at least one-half the slit width and that the extent of the near-field increases with the slit width. Further, the calculations indicated that the near-field energy flux exhibited a close-to-exponential decrease in intensity with increasing distance from the screen. These results suggested that rigid stability requirements would be needed in the direction perpendicular to the surface of the object and of the screen in order to obtain reproducible results.
As described in the aforesaid copending application Ser. No. 520,041, an aperture plate incorporating apertures having diameters on the order of 300 Angstroms has been constructed, and it has been demonstrated that visible light can pass through such apertures, independently of the wavelength of the light. Relatively high transmission is obtained, sufficient to obtain detectable amounts of light using an ordinary microscope illuminator lamp as the light source.
Again, as set forth in Ser. No. 520,041, it was found that spectral phenomena, produced by illuminating an object, also exhibit a near-field radiation pattern; that is, spectral phenomena emanating from an object are essentially perpendicular to the surface from which they emanate, within the near-field region of an aperture used to image the surface. This phenomenon, combined with the use of extremely small apertures, permits observation, in the near-field of an object, of a field of view which is limited to the area of the aperture projected on the surface being observed. As long as the surface is within the near-field of the aperture, the spectral phenomena passing through the aperture will be collimated. An image of the object can be formed if the aperture (or an aperture array) is scanned in a raster-like fashion relative to the object. Such a scanning system has a spatial resolution limited by the aperture diameter instead of by the wavelength of incident light or the spectral phenomena emanating from the surface, and thus can have a resolution on the order of one-tenth to one-sixteenth the wavelength of the incident light.
Although the aperture or aperture array of Ser. No. 520,041 works well as described therein, it has been found that some difficulty has been encountered in attempting to obtain precise observations in those cases where the surface of the object is uneven or is shaped in such a way as to prevent the aperture from being positioned so that the object lies in the near-field of the aperture. This condition severely limits the depth of field of the imaging device, and is of particular concern in the study of biological specimens where it may be desirable to view an object having an extremely uneven surface while still avoiding injury to that object.
When viewing objects through extremely small working distances, the positioning of the viewing aperture becomes extremely critical. In such cases it becomes essential that the instrument be isolated from any environmental vibrations, that changes in materials due to thermal drift be prevented or compensated, and that extremely precise positioning of the aperture be available in all three spatial dimensions. Further, since the light transmitted through a submicron aperture is weak, a sensitive detection system is extremely important, and care must be taken to reduce noise.